Reaction wheels are commonly used to control the attitude (or orientation) of a spacecraft, satellite, or the like. A reaction wheel typically consists of a rotor (or wheel) and an electric motor, wherein the electric motor is operated to apply torque to increase or decrease the angular velocity of the rotor, and thereby alter the angular momentum of the rotor. This change in angular momentum produces a reactionary torque which causes the spacecraft to rotate to the desired attitude or orientation. Attitude control systems (ACSs) and other spacecraft orienting applications often utilize a momentum control system (MCS) that includes at least three reaction wheels to provide rotation about three-different axes.
The bearings of reaction wheels exhibit friction, which impair operation of reaction wheels at low speeds or when a reaction wheel changes its direction of rotation. For example, when the rotor changes its direction of rotation, forces caused by static friction increase the amount of torque required to cause the rotor to resume rotating in the desired direction. Closed-loop control systems have been developed to mitigate the friction in reaction wheel bearings. However, due to nonlinearities of the frictional forces in the bearings, existing control systems fail to adequately compensate for frictional forces within a desired amount of time, resulting in attitude errors.